Aluminium is used extensively for lightweight structures such as automotive and aerospace components where a combination of strength and corrosion resistance is essential. Aluminium owes its inherent corrosion resistance to a naturally occurring passive oxide which forms on the metal when exposed to the atmosphere. The thickness of the oxide layer is in the nanometer range which limits the performance of the metal against extreme mechanical and chemical attack. Electrochemical processes have been investigated with a view to producing coatings on such metals to enhance the strength and corrosion resistance of the metals.
Anodising is a well known electrochemical process for coating metals whereby a metal component, such as an aluminium work piece, for example, is submerged in a bath of an electrolytic solution. The work piece to be coated acts as a positive electrode and a direct current is applied. This results in an anodic coating comprising a porous layer of aluminium oxide being formed on the work piece. The thickness of the aluminium oxide is increased by the anodising process through an electrochemical reaction in acidic electrolytes such as sulphuric, phosphoric or oxalic acids. The process is commonly used to increase corrosion resistance and adhesion properties of the aluminium surface for a variety of applications.
The anodised aluminium oxide layer is nanoporous in structure with a self-assembled, hexagonal array of pores extending from the surface of the oxide to a thin barrier layer at the metal-metal oxide interface. The oxide growth and nanopore formation mechanism is a result of flow of anodic alumina in the barrier layer region due to the combination of growth stresses and field assisted plasticity. The stresses that drive the flow of material are due to electrostriction of the oxide layer which is plasticised under the electric field. The flow of material proceeds from the barrier layer into the pore walls forming Al2O3 columns in a self-assembled structure.
The anodic coating forms part of the metal but it has a porous structure which enables further treatments to be applied. For example, top coats and lacquers may be incorporated in the coating. Following the anodising process, the pores of the anodic layer need to be closed. If the pores are not sealed, the surface could have poor corrosion resistance.
For anti-corrosion applications, sulphuric acid anodising (SAA) is most commonly employed. A known significant advantage of SAA anodic layers is, for example, the ability of the pores of the anodic layer to close by surface hydration resulting in improved barrier properties thereby providing corrosion resistance. Hydration on the SAA surface proceeds rapidly after anodising and can be accelerated by hydrothermal treatment to achieve increased corrosion protection while also entrapping any applied inhibitors or dyes. Both natural and hydrothermally induced hydration results in pore blocking near the surface of the anodised layer. Hydration continues naturally over time as the pore closing effects move down the pore channel towards the metal surface. This continued hydration, termed “auto-sealing”, results in an increase in the barrier properties of the anodic layers even during exposure to aggressive environments. Such a feature is responsible for the excellent long term and accelerated corrosion resistance of sulphuric acid anodised layers on copper-free wrought alloys.
However, in the case of copper-containing alloys, the protective properties provided by anodic layers formed by sulphuric acid anodising is reduced by the inclusion of copper ions within the oxide network. The presence of copper, as well as the random orientation of the pores, leads to difficulties with hydration sealing. To improve the corrosion protection on copper containing alloys, anodising processes have been developed including boric-sulphuric (BSAA) and tartaric-sulphuric (TSAA) acid anodising for corrosion and adhesive bonding applications.
Chromate based anodising processes and sealing processes are generally regarded as the target performance benchmarks for any developed anodising technology. However, due to the carcinogenic nature of these materials, the use of chromate based processes are currently restricted or being eliminated from anodising industries.
Anodising procedures currently used in the art include the use of mixed tartaric sulphuric acid (TSA) which has been shown to produce corrosion resistance and fatigue resistance equivalent to chromic acid anodising. However, on the other hand, due to surface hydration and small pore size of the resulting oxide layer, the adhesion of top coats and lacquers has been found to be inferior to that achieved using chromic acid anodising.
The conventional phosphoric acid anodising process is well known as having excellent adhesion properties, comparable to chromic acid anodising. However, this treatment imparts extremely poor corrosion resistance to the metal.
In order to achieve a balance of adhesion and corrosion resistance, duplex anodic layers have been investigated.
International Publication No. WO 2006/072804 relates to a method for the formation of anodic oxide films on aluminium or aluminium alloys. The anodic oxide coating disclosed in WO 2006/072804 is suitable for adhesive bonding of aluminium alloy structures. A duplex anodising procedure is described which involves the use of a mixed sulphuric phosphoric acid anodising step followed by a sulphuric acid treatment. The mixed bath is used to achieve a balance between hydration resistance and anodising voltage. However, in the process disclosed, the voltage used for the first anodising step is limited due to the mixture of acids used. In particular, when anodising in the presence of sulphuric acid, a lower voltage must be used compared to that used when anodising in the presence of phosphoric acid. The voltage used in the anodising step described in WO 2006/072804 is limited due to the mixture of sulphuric acid and phosphoric acid. The process disclosed in WO 2006/072804 also suffers from the disadvantage that the duplex anodic layer formed is not optimised for adhesion as the pore size is relatively small. In order to prevent pore closure due to hydration and accordingly to retain the adhesion properties of the surface, a system comprising pores having a large diameter is required.
A technology similar to that disclosed in WO 2006/072804 is described in US20050150771 in which, again, the initial anodising procedure requires a mixed sulphuric phosphoric acid anodising electrolyte to achieve lower forming voltage. It is notable that the forming voltages are limited to below 25V. However, optimum surface adhesion is not achieved as this can only be provided by sulphate free anodised layers formed under larger potentials. Thus, again, the duplex anodic layer formed is not optimised.
Thus, despite the development of anodising treatments for copper rich aluminium alloys, the corrosion protection afforded by the anodic layers is limited and does not provide the desired corrosion resistance.
In addition, many aerospace and automotive companies are utilising sol-gel chemistries as a replacement for hexavalent chrome anodising and conversion coatings. For corrosion resistance of anodised aluminium using sol-gel based sealers, the combination of both natural hydration of the surface as well as penetration of the sol-gel into the pores of the anodic is required for full performance. However, there are some inherent problems associated with the combination of sol-gel chemistry and current anodising processes. Migration of sol-gel materials into the aluminium oxide pores can also be limited.
Accordingly, there is a need for an improved method for the production of anodic coatings which are capable of imparting desirable corrosion resistance as well as the desirable adhesion and abrasion properties to an anodisable metal. Furthermore, the anodic layer requires optimisation in order to achieve full encapsulation of materials applied to the anodic layer(s) such as sol-gel sealers without affecting the desired properties of the anodic layers. Such optimised corrosion resistance; and optimised adhesion and abrasion properties as well as optimised for achieving full encapsulation of applied materials is not achieved by the known processes.